Fluorescence via Reverse Intersystem Crossing from Higher Triplet States in a Bisanthracene Derivative

TL;DR

This study investigates the high quantum efficiency of a bisanthracene derivative in OLEDs by analyzing vibronic couplings and proposes a new fluorescence mechanism involving reverse intersystem crossing from higher triplet states.

Contribution

The paper introduces the FvHT mechanism, explaining fluorescence via reverse intersystem crossing from higher triplet states, applicable even in asymmetrical molecules like BD1.

Findings

FvHT mechanism involves T4 to S2 RISC conversion

BD1 achieves FvHT despite asymmetry

A general condition for suppressing non-radiative transitions

Abstract

To elucidate the high external quantum efficiency observed for organic light-emitting diodes using a bisanthracene derivative, BD1, as the emitting molecule, off-diagonal vibronic coupling constants (VCCs) between the excited states of BD1, which govern non-radiative transition rates, were calculated employing time-dependent density functional theory. The VCCs were analysed based on the concept of vibronic coupling density. The VCC calculations suggest a fluorescence via higher triplets (FvHT) mechanism, which entails the conversion of a T$_4$ exciton generated during electrical excitation into an S$_2$ exciton via reverse intersystem crossing (RISC); moreover, the S$_2$ exciton relaxes to a fluorescent S$_1$ exciton because of large vibronic coupling between S$_2$ and S$_1$. This mechanism is valid as long as the relaxation of triplet states higher than T$_1$ to lower states is suppressed. The symmetry-controlled thermally activated delayed fluorescence (SC-TADF) and inverted singlet and triplet (iST) structure, which have been proposed in our previous studies, are the special examples of the FvHT mechanism that need high molecular symmetry. However, BD1 achieves the FvHT mechanism in spite of its asymmetrical structure. A general condition for the suppression of radiative and non-radiative transitions in molecules with pseudo-degenerate electronic structures such as BD1 is discussed. A superordinate concept, fluorescence via RISC, which includes TADF, SC-TADF, iST structure, and FvHT is also proposed.